Dual-transformer self-oscillating converter

　　In the dual-transformer self-excited oscillating converter, the switching function of the 6V power adapter is controlled by the saturation of the small drive transformer instead of the main power transformer, which improves the performance and makes the power transformer have better efficiency.

　　Since the main power transformer is no longer saturated, the shape of the B/H hysteresis loop is not the key, and a more optimized transformer design becomes possible. Furthermore, since the collector termination current is lower, the power transistor is turned off under more controlled conditions, so the switching effect of the transistor is better. The switching frequency is also more constant, because the drive transformer has no output load, and the voltage across the drive transformer is not directly related to the input voltage. Therefore, its operating frequency is not sensitive to changes in the power adapter and load.

　　Since all these aspects are better than simple saturation converters, dual transformer converters are commonly used in high-power applications. Commonly used in low-cost DC (direct current) transformers.

　　Two main shortcomings of the simple self-oscillating two-transformer square wave converter limit its application.

　　(1) Since the pulse width is not easy to decrease when it starts to conduct, it is difficult to realize soft start.

　　(2) Since it is a square wave (100% duty cycle) output, the output is not adjustable.

　　The camera power adapter closes the switch, and the current flowing through R1 makes one of the two drive transistors turn on, assuming that Q1 is on (this depends on the gain of the two components and the respective base-emitter voltage). With the conduction of Q1, the independent drive transformer T: the generated drive current provides regenerative feedback. The primary side of Ts is current coupled with the collector of Q1, and it provides base drive for Q1 through the secondary winding T2s. The phase setting of the winding should make the collector current of Q1 increase and cause the base current of Q1 to increase. Since T: is a current transformer, the base drive current is determined by the turns ratio of the collector current and T: (the turns ratio used in this example is 1/5, and the current amplification factor is 5).

　　When Q1 is in the conducting state, the voltage V across the secondary side of the drive transformer Ts is the base-emitter junction voltage of Q1 plus the tube voltage drop of D (about 1.3V in total), and the voltage across the drive winding T of Q2 is equal to The voltage values at both ends of T2s are the same, but in opposite directions, making the base voltage of Q2 a negative 0.7V.

　　After a certain period of time determined by the magnetic core area and the secondary voltage V, the drive transformer T2 will be saturated, the drive voltage of Q1 will drop to zero, and Q1 will turn off. This process occurs before the main transformer T1 is saturated.

　　As Q turns off, its collector current will drop to zero, and the voltage on T2 will be reversed due to flyback. Q2 turns on and Q1 turns off completely. The same working cycle as before appears on the quality tube Q2, and its collector current flows through the other half of the winding of the driving transformer Tm and the main transformer Tin, and is reversed.

　　Since the forward voltage of Q1 base-emitter junction and D or the forward voltage of Q2 base-emitter junction and D1 can determine the voltage across the secondary winding of T2, the synchronous voltage of T2 is fixed and independent of the power adapter voltage. Therefore, the operating frequency is largely independent of the change of the power adapter and load. However, temperature changes will affect the two diodes and two base voltages and the saturation flux density of the driving transformer T. Therefore, the frequency is still affected by temperature changes. In many applications, small changes in frequency are not important.

　　Clamping diodes D2 and D2 are connected to both ends of the switching transistor's collector and emitter junction to provide a path for the reversed magnetizing current. Otherwise, when the converted load current is less than the magnetizing current, the magnetizing current will flow from the base of Q1 and Q2 to collector. Although this only occurs under very light loads, since it is driven proportionally, it can also cause problems and should be further observed.

　　Consider the moment before Q1 is turned off: the transformer magnetization current has been established at the primary side of the power transformer T and the collector of Q, and the direction of flow in the winding is from right to left. When Q1 is turned off, the magnetizing current in the winding attempts to continue to flow in the same direction to make the collector of Q1 positive and the collector of Q2 negative (flyback).

　　Since the magnetizing current cannot drop to zero immediately, and Q1 cannot flow to the collector of Q1 after being turned off, the circuit will be changed from the collector of Q1 to the collector of Q2, passing through the clamping diode D2, and flowing into Tm from right to left, if Without D3, the current will flow to the base-assembly of Q2. Since the current flow direction is opposite to the normal collector current direction, the base drive current leaves the base-emitter junction and enters the base-assembly (transistor reverse bias).

　　This reverse collector current also flows through the primary side of the drive transformer T, but its direction is opposite to the direction required by the positive feedback. This reverse current will prevent or at least delay the conduction of Q2, which is undesirable. However, a low-impedance path provided by the clamp diode D2 can shunt most of the reverse current from Q1. In some cases, it is required to further connect blocking diodes in series with the collectors of Q and Q2.

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